In a small saucepan, cook potatoes and celery until tender; add frozen vegetables and cook for 5 minutes; drain. Sauté onion in butter until tender but not brown. Blend in flour and salt, browning slightly. Add broth all at once. Cook and stir until thick and bubbly.
Add chicken and cooked vegetables. Stir in celery salt; set aside. Prepare pastry for bottom of pie pan. Trim edges. Pour chicken filling into prepared pie shell. Top with remaining pastry. Seal edges and flute. Cut steam vents in top. Brush top pastry with milk. Bake at 400 degrees for 20 to 25 minutes.